When an X – ray hits a material, it generally penetrates the material and it's energy is gradually reduced as it passes through the matter. There are several mechanisms via which this may happen, summarised in the table below.
Note that
= attenuation coefficient, such that in a distance
a fraction
of the energy of the X - ray iremains unattenuated.
= atomic number of the atoms of the material.
= energy of X – ray photon.
Attenuation Mechanism | Energy Range of X – ray Photons (MeV) | Fate of X – Ray Photon | Relationship Between | Relationship Between |
Photoelectric Effect | Less than 0.1 | Photon disappears. An electron is ejected from inner orbitals of atom. |
|
|
Compton Scattering | 0.5 – 5.0 | Photon is inelastically scattered by atomic electron, and loses some energy. |
| Decreases slowly with |
Particle – Antiparticle Pair production | Greater than 1.02 | Photon disappears and produces electron – positron pair. |
| Rises slowly with |
